The circuit of the present invention relates generally to means for controlling the duty cycle of a power supply. More specifically, the circuit of the present invention relates to means for controlling the duty cycle of a two-stage electrostatic precipitator power supply transformer in response to high voltage transients generated by arcing between components of the precipitator and reflected therefrom through the secondary winding of the transformer to the primary winding.
The use of two-stage electrostatic precipitators to remove dry particulates or contaminants from an air stream is well known in the art. Typically, an ionizer is utilized to produce an electrostatic field to charge the contaminating particles. The charges particles are then passed through a collecting cell comprised of charged and grounded plates to accumulate the charged particles or contaminents.
In order to achieve efficient operation of electrostatic precipitators of the type described above it is well known that relatively high voltage levels must be utilized in both the ionizer and collecting cells. As contaminants are accumulated by the precipitator collecting cells, it is not uncommon for intermittent arcing, which may progress to a virtually continuous condition, to occur between the highly charged potential surfaces and ground within the precipitator. Arcing of this type results in the generation of high voltage transient spikes which are fed back from the precipitator to the power supply secondary winding and are therefrom reflected into the transformer primary circuit. And, since the components typically used in power supplies intended for use with electrostatic precipitators are normally not rated to withstand the high voltage transient spikes, continued arcing within the precipitator may significantly shorten the useful service life of the power supply, as well as significantly damaging the precipitator itself.
Prior art means for protecting electrostatic precipitator power supplies generally takes one of three forms. Frequently a fuse or circuit breaker is used in the transformer primary circuit in order to protect the power supply components from destructively high current levels. Also, it is known in the prior art to employ ferroresonant circuits in the transformer secondary to protect against a continuous short circuit condition. Although some of the foregoing devices present somewhat of a nuisance, they generally provide adequate power supply protection for electrostatic precipitators utilized in most residential and commercial applications. However, in industrial applications where higher operating voltages and larger contamination accumulations of varying conductivity result in more frequent arcing between precipitator components, the prior art power supply protection devices do not provide acceptable performance. In other words, although the prior art devices provide protection against conditions such as a complete short circuit, for anything between normal operation and the short circuit, the protection is inadequate. And, significantly, the prior art devices provide virtually no protection in the case of intermittent or continuous arcing between precipitator potential surfaces.
It will be appreciated that any protective device designed to overcome the foregoing limitations in the prior art will most advantageously be located in the power supply primary circuit. This obviates the need for the additional isolation which would be necessary if the device was located in the secondary circuit due to the high voltage levels developed therein.